1. Field of the Invention
The present invention relates to an asynchronous transfer mode (ATM) private network. Specifically, this invention is a telephone service system in ATM which implements telephone service in the ATM by converting an analog telephone signal, which are input/output to/by a switch in a public switched telephone network (PSTN), into an ATM cell.
2. Discussion of Related Art
Wide-band integrated services digital network (ISDN) is a communications network which integrally provides telephone service, video telephone, data transmission, and facsimile. In the ATM, the units of data having a fixed length, called cells, are transferred and switched, thus the ATM is appropriate for the wide-band ISDN. The ATM method is not used in all areas, so ATM networks, which may be need to be connected to each other. U.S. Pat. No. 5,459,722 relates to an interface device for connecting ATM networks, which are far away from each other, using an existing PSTN.
A digital trunk transmission method used in the PSTN for a usual telephone service employs a time division multiplexing (TDM) system. In the TDM system, time slots, which are repeated with predetermined intervals, are allocated to corresponding subscribers. Information is loaded into the slots and transmitted. The multiplexed information is discriminated by the position of the slot in the temporal domain. If the information transmission speed changes, a slot which is capable of covering the maximum speed of the information is required, so a subscriber is allocated a time slot, regardless of the existence of data to be transmitted. Consequently, the average transmission speed of information is very low, and the transmission lines are not used efficiently.
A conventional method for a telephone service exchanges subscribers' telephone signals through a switch and PSTN to provide the signals to subscribers. A digital trunk transmission method for the conventional telephone service always allocates time slots to each subscriber, regardless of the existence of data to be transmitted, thus overall transmission bandwidth becomes large. For implementing multimedia service, enormous transmission bandwidth is required to accommodate the services subscribed by users. To solve this problem, an ATM method is presented. However, the existing PSTN cannot perform high speed transmission because of high loss, so the PSTN cannot be correlatively linked with the ATM method.